1. Field of the Invention
The present invention relates to the use of a moire pattern to determine the position of a view point relative to a predetermined reference point in three dimensional space. The invention also relates to a means for designing the moire pattern such that differing degrees of sensitivity may be determined relative to the reference point and the moire pattern.
2. General Background
The use of moire patterns to determine position relative to a plane of viewing has long been known in the prior art. Moire patterns are generally formed using two "templates," each of which has a grid pattern of alternating light and dark areas. The template patterns are aligned and placed one behind the other. When viewed form a view point in which the template patterns are not aligned, the viewer will perceive a superposition pattern of large alternating light and dark zones arising from the interference of the patterns. These patterns are coarser than the actual grid pattern and represent the low or "beat" frequency of the interference. As a viewer out of alignment with the plane of the moire templates, the beats will appear to "run" indicating the misalignment of the viewer with the plane of the templates.
3. Prior Art
This phenomenon has been utilized in a number of devices to determine position relative to a plane. U.S. Pat. No. 3,604,813 describes the use of moire patterns in a device known as a "planoscope." This invention disclosed the use of two moire grids lying in intersecting planes which were perpendicular to a vertical grid pattern of differing pitch for each of the grids. The invention created an image behind the planoscope which would indicate whether the viewer was to the left or right of the plane bisecting the center of the moire grids. The invention also disclosed that a party could estimate the distance form the planoscope by a determination of the number of beats the viewer perceived. Thus, the viewer was provided with visual information relating to two dimensions: lateral position with respect to the bisecting plane and distance from the planoscope However, the invention did not indicate how to determine distance from the planoscope as a function of the number of beats. Further, the invention required that the moire grids be fixed with respect to each other.
U.S. Pat. No. 4,166,699 disclosed another planoscope structure which utilizes three moire grids to determine view point position relative a plane perpendicular to the moire grids. The invention utilized three planar grids to create a moire pattern which would indicate whether the viewer was above or below the perpendicular plane. The sensitivity of the pattern could be determined by the pitch of the grid and the angle formed between the patterns. As with U.S. Pat. No. 3,604,813, a viewer could infer the distance form the planoscope as a function of the number of beats perceived. Again, the grids maintained a fix position with respect to each other.
U.S. Pat. No. 4,245,912 disclosed a similar structure comprising three planar moire grids enclosed in a parallelpiped box with mirrors to permit improved viewing utilizing ambient or artificial light. As in U.S. Pat. No. 4,166,699, the moire patterns provided visual information relative to a perpendicular plane passing through the three moire grids. Further, a viewer could again infer distance to the planoscope as a function of the number of beats perceived.
U.S. Pat. No. 3,957,378 disclosed a moire pattern transducer which gave the viewer alignment information in two dimensions. The invention disclosed the use of grid patterns comprising orthogonal line patterns and an index piece The invertion was used to detect alignment and recited a structure which called for a controlled rotational movement of the masks relative to each other. Further, detection of alignment position was accomplished by means of a plurality of photodetectors mounted essentially above the grid patterns. The invention permitted the user to accomplish alignment in essentially the X-Y plane but did not provide the user information related to the distance from the patterns generated.
U.S. Pat. No. 4,572,952 utilized moire pattern detection by a charged coupled device ("CCD") to determine linear or angular position. The invention disclosed the use of a single mask having grid patterns of differing pitches which created the moire pattern. The invention called for a partly moving transmissive mask with scales of varying pitch which created a moire pattern detected by a two dimensional CCD which permitted accurate interpolation of the angular position of a shaft or linear position. The invention did not provide information relating to position detection in three dimensional space. Further, the detector for the structure (the CCD) had to remain in close proximity to the mask.
It will be appreciated that none of the above inventions provided a suitable means of determining three dimensional position relative to a predetermined point. Further, while U.S. Pat. No. 4,572,952 provided information relative to rotational or angular positioning, it was incapable of providing orthogonal three dimensional information as well.